Indirect opthalmoscopy, in particular binocular indirect opthalmoscopy, is a procedure commonly used to examine the fundus of an eye. The technique involves using a hand-held condensing lens which is placed between the examiner or user and the patient. This lens produces an aerial image of the retina that is viewed through the observation system of the opthalmoscope. The lens images the exit point of the illumination system and entrance point of the observation system in the pupillary plane of the patent's eye, thus ensuring a wide field of retinal illumination together with a wide field of observation.
The indirect opthalmoscope housing comprises a light source from which a beam is projected towards the eye via a plane mirror, angled such that the light passes through the hand held condensing lens before entering the eye. The condensing lens brings the light to a focus in the pupillary plane of the eye being examined. As a result of this arrangement, the paths of the illuminating and observation beams can be separated as they pass through the pupil, thus making the system capable of producing a reflex free view of the retina.
The opthalmoscope typically utilises a headband that enables the device to be mounted on the examiner's head. It also includes viewing optics through which an examiner can obtain a binocular view of the image of the fundus. The two viewing axes and the illumination light path must be able to pass through the pupil without interference.
For maximum stereopsis, the right and left viewing axes must be separated as far as possible from each other. The illumination beam, which must be focussed in the pupillary plane, should be positioned as near as possible to the pupil margin. This enables the illumination and observation paths to be separated as they pass though the pupil, thus decentring and minimising reflections from the cornea and the crystalline lens. Dilating the patient's pupil by means of a drug facilitates this process, but this may not always be possible.
The diameter of the illumination light beam can be varied by using a range of different light stops. Generally, the larger the pupil, the larger the diameter that is used.
Earlier instruments had viewing optics, illumination mirror and light source acting independently from each other. In addition, a range of illumination patch sizes is available for the examiner to select. This meant that the examiner was faced with a series of adjustments to make to achieve optimum viewing through a specific pupil size.
U.S. Pat. No. 4,449,767 attempted to overcome this by having two viewing mirrors arranged on a wedge shaped platform which could be moved towards or away from the patient's eye. The illumination mirror was mounted separately.
Heine et al. in U.S. Pat. Nos. 4,684,227 and 5,223,863 have attempted to overcome this further by mounting two viewing mirrors and a tilting illumination mirror on a common platform that can be moved toward and away from the object being viewed. This adjusts the position of the viewing and illumination axes, but a further adjustment to select optimum beam size is still required.
Welch Allyn in U.S. Pat. No. 5,394,201 achieve a similar result by having a pivoting illumination mirror connected to reflecting mirrors in a sliding carriage so that the axis of the light beam and the axes of the viewing paths move closer together or apart as the carriage moves in one direction or the other.
In all the above quoted patents which link the separate viewing and illumination axes, a further adjustment to select optimum beam size is still required.
Accordingly it is desirable to provide an improved binocular opthalmoscope for indirect observation of the eye that is easier for the examiner or user to adjust for optimum use.